The present invention relates to an electron gun assembly for a cathode ray tube, used in a color image receiving tube (television picture tube), and more specifically, to an improved structure of an electron gun assembly for a cathode ray tube.
In general, a color image receiving tube has, as shown in FIG. 1, an outer enclosure consisting of a panel 1 and a funnel 2. On the inner surface of the panel 1, a phosphorous screen made of three-color phosphorous layers which emit blue, green and red light rays, namely, a target 3 is formed. A shadow mask 4 is provided on a further inner side of the panel, so as to face the phosphorous screen 3. Also, an electron gun assembly 7 for emitting three types of electron beams 6B, 6G and 6R is provided in a neck 5 of the funnel 2. In the tube, as three electron beams 6B, 6G and 6R emitted from the electron gun are deflected by vertical and horizontal deflecting magnetic fields which are generated by a deflecting device 8 mounted on an outer side of the funnel 2, the phosphorous screen 3 is scanned vertically and horizontally by these three electron beams 6B, 6G and 6R via a shadow mask 4, thus displaying a color image.
In connection with the color image receiving tube described above, the self-convergence-inline type color image receiving tube is widely used in practice. The color image receiving tube of this type has the following structure. That is, an inline type electron gun assembly is employed, and which emits three electron beams 6B, 6G and 6R arranged in a line, which are specifically, a center beam 6G and a pair of side beams 6B and 6R running on the same horizontal plane. These three electron beams 6B, 6G and 6R arranged in the line are deflected by a pin-cushion shaped horizontal deflection magnetic field and a barrel shaped vertical deflection magnetic field, which are generated by a deflection device 8 to which a deflection signals is supplied from a deflection unit (not shown). Therefore, the three electron beams 6B, 6G and 6R can be converged without providing a special converging device.
However, in connection with the self-convergence inline type color image receiving tube, the vertical and horizontal deflection magnetic fields generated by the deflection device 8 have pin-cushion shape and barrel shape, respectively, which are not uniform within themselves. As a result, the three electron beams 6B, 6G and 6R are affected by these non-uniform magnetic fields to create astigmatism, and therefore at the peripheral portion of the screen, they are strongly converged in the vertical direction. Consequently, at the peripheral portion of the screen, the beam spot is made to have an elliptical shape elongated in the horizontal direction, including a core portion having a high luminance and a halo portion having a low luminance located in the vertical direction to the core portion. Such a phenomenon deteriorates the resolution at the peripheral portion of the screen.
As a solution to the deterioration in the resolution at the peripheral portion of the screen, an electron gun assembly of the dynamic focus type, has been proposed, which has a structure in which quadruple-pole lens is formed in the main electron lens unit for focusing an electron beam. In this electron gun assembly, an electron beam generating unit, a quadruple-pole electron lens and an ultimate focusing lens are formed in the order in the traveling direction of the electron beams, that is, along the direction from the cathodes towards the phosphorous screen. With this structure, a dynamic focus voltage which varies in synchronism with the deflection of the electron beam, is supplied to an opposite electrode which constitute the quadruple-pole lens. Thus, the electron beams traveling towards at the peripheral portion of the screen are diverged in the vertical direction, and converged in the horizontal direction. Further, the horizontal- and vertical-directional focusing effect of the ultimate focusing lens is weakened, thus suppressing the deterioration in the resolution at the peripheral portion of the screen. The electron gun assembly of this type entails the problem of a withstanding voltage of the voltage supply unit since two types of medium voltages must be supplied to the opposite electrode from an external device in the quadruple-pole lens.
Jnp. Pat. Appln. KOKAI Publication No. 1-232643 and U.S. Pat. No. 4,945,284 each provide a solution to the above-stated problem, and they disclose an electron gun assembly having the following structure such as shown in FIG. 2. That is, the electron gun assembly has cathodes K, and first to seventh grids G1 to G7 which are arranged in the order from the cathodes K towards the phosphorous screen. Further, a resistor 10 is provided adjacent to the fifth and sixth grids G5 and G6 which constitute a quadruple-pole lens QL, so as to connect the fifth and sixth grids G5 and G6 via the resistor 10, thus supplying a voltage to the fifth grid G5 via the resistor 10.
However, the cathode ray tube having the above-described structure entails the following problem. That is, in the color image receiving tube shown in FIGS. 1 and 2, a anode voltage of 23 to 35 kV is supplied to the seventh grid G7 via a conductive film 12 applied on over an area from an inner surface of a portion of the funnel 2, having a larger diameter, to an inner surface of the adjacent portion to the neck 5, a valve spacer 13 brought into elastically contact with the conductive film 12 and a shield cup C. A focusing potential, which is about 20 to 35% of the anode voltage is applied to the sixth grid G6 via a stem pin 14 which air-tightly pierces through the end portion of the neck 5.
In some cases, the inner surface of the neck 5 is charged under the influence of the conductive film 12, and an electric field emission phenomenon may occur from a grid which constitutes the electron gun assembly. If an electron emission occurs in the sixth grid G6 due to the electric field emission phenomenon, the focusing potential is varied, resulting in that the electron beam is not focused in an optimal fashion. Consequently, the resolution is lowered, and the image quality is deteriorated. Further, a discharge may occur due to the electron emission, and the discharge current or noise thus created may damage the electrical circuit of the cathode ray tube apparatus, or may cause a malfunction of the computer connected thereto.
In order to avoid such a problem, the following measures have proposed. That is, a suppressor ring made of a metal wire is set so as to surround an insulation support rod 16 for fixing the electrodes of the electron gun assembly into an integral body, and then the suppressor ring is evaporated by heat to form a metal deposition film on the inner surface of the neck 5 in a manufacturing step of the color image receiving tube, thereby stabilizing the potential of the inner surface of the neck 5.
However, as can be seen in FIG. 2, with regard to the electron assembly gun having the structure in which a resistor 10 is placed adjacent to the fifth and sixth grids G5 and G6, it is sometimes very difficult to dispose the suppressor ring at an effective position.
More specifically, in the case where the suppressor ring is placed between the terminal 17a of the resistor 10 and the shield cup C, for example the sixth grid G6, the metal deposition film is formed adjacent to the conductive film 12, and therefore the potential of the inner surface of the neck 5 cannot be decreased. In the case where the suppressor ring is disposed between terminal 17a and terminal 17b of the resistor 10, parts of the metal deposition film may be attached to the resistor 10 located near-by, which creates a short-circuit between both terminals 17a and 17b of the resistor 10. For this reason, the suppressor ring cannot be provided at the above positions. On the other hand, in the case where the suppressor ring is provided close to the cathodes K rather than to the resistor 10, the metal deposition film is attached partially onto the inner surface of the neck 5 at a low potential, and therefore the potential of the inner surface of the neck 5 can be stabilized. However, the portion of the inner surface of the neck 5, which is close to the sixth grid G6, is located near the conductive film 12, and therefore the potential cannot be sufficiently lowered. The effect of suppressing the electric field emission from the sixth grid G6, the terminal 17a of the resistor 10 connected to the sixth grid G6, or the like, may not become satisfactory.